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Is it possible to add a key to a Python dictionary after it has been created? It doesn't seem to have an .add() method...

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9 Answers

up vote 370 down vote accepted 
>>> d = {'key':'value'}
>>> print d
{'key': 'value'}
>>> d['mynewkey'] = 'mynewvalue'
>>> print d
{'mynewkey': 'mynewvalue', 'key': 'value'}
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4  
59  
Heh, now I feel kind of stupid. – lfaraone Jun 21 '09 at 22:15
147  
30,000 views makes it not so stupid – Yarin Jan 20 '12 at 19:56
11  
Does not (directly) work with nested dictionaries: test['x']['y']. test['x'] has to be initialized first. – koppor Jul 22 '12 at 10:35
5  
137000 now... – TakeS Feb 10 at 15:25
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up vote 18 down vote 
dictionary[key] = value
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2  
You were the first. But unfortunately, not your answer on the top. – Vanuan Nov 4 '12 at 13:36
up vote 41 down vote

Yeah, it's pretty easy. Just do the following:

dict["key"] = "value"
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1  
You've posted at the exactly same time. But unfortunately Paolo's answer has more votes. – Vanuan Nov 4 '12 at 13:39
up vote 138 down vote 
>>> x = {1:2}
>>> print x
{1: 2}

>>> x.update({3:4})
>>> print x
{1: 2, 3: 4}
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13  
That's the answer that should be in the top. – Vanuan Nov 3 '12 at 19:56
this applies for LISTS, not for dictionaries – reiven Nov 9 '12 at 19:59
1  
What do you mean? 'update' perfectly works for dictionaries. – Vanuan Nov 12 '12 at 20:21
2  
This answer was exactly what I was looking for. And re: "update" - docs.python.org/2/library/stdtypes.html#dict.update – Matt Dec 7 '12 at 22:05
up vote 39 down vote

I feel like consolidating info about python dictionary :

#### Making a dictionary ####
data = {}
# OR #
data = dict()

#### Initially adding values ####
data = {'a':1,'b':2,'c':3}
# OR #
data = dict(a=1, b=2, c=3)

#### Inserting/Updating value ####
data['a']=1  # updates if 'a' exists, else adds 'a'
# OR #
data.update({'a':1})
# OR #
data.update(dict(a=1))

#### Merging 2 dictionaries ####
data.update(data2)  # Where data2 is also a dict.

Feel free to add more !!

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3  
Everything you need is in docs.python.org/library/stdtypes.html#mapping-types-dict – Michael Hoffman Dec 5 '11 at 6:12
3  
Though I read the docs I didn't notice dict.update until I saw an example here with dict.update({'a':1}). The docs list update as update([other]) which I guess never caught my eye. – Matt Dec 7 '12 at 22:08
1  
@Yugal Great job putting it all together :) Will definitely help beginners. – UGS Mar 27 at 10:10
up vote 6 down vote 
data = {}
data['a'] = 'A'
data['b'] = 'B'

for key, value in data.iteritems():
    print "%s-%s" % (key, value)

results in 

a-A
b-B
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this answer is not much different than already provided – Vanuan Nov 3 '12 at 19:53
@Vanuan, the code is different though – daydreamer Nov 4 '12 at 7:54
stackoverflow isn't about the code, it's about solutions to problems. Your proposed solution (use index assignments) is exactly the same as already provided. – Vanuan Nov 4 '12 at 13:30
up vote 5 down vote

Also, if you want to add a dictionary within a dictionary you can do it this way...

Example: Add a new entry to your dictionary & sub dictionary

dictionary = {}
dictionary["new key"] = "some new entry" # add new dictionary entry
dictionary["dictionary_within_a_dictionary"] = {} # this is required by python
dictionary["dictionary_within_a_dictionary"]["sub_dict"] = {"other" : "dictionary"}
print (dictionary)

Output: {'new key': 'some value entry', 'dictionary_within_a_dictionary': {'sub_dict': {'other': 'dictionarly'}}}

NOTE: Python requires that you first add a sub dictionary["dictionary_within_a_dictionary"] = {} before adding entries... this is kinda a bug with python.

Hope that helps...

V$H.

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3  
this is as irrelevant to the question asked as most of the comments in php.net manual pages... – Erik Allik Jun 1 '12 at 21:05
3  
How is this a bug? – Kugel Aug 2 '12 at 19:05
2  
This is not a bug. – Vanuan Nov 3 '12 at 19:54
python inception – TMP Mar 11 at 10:09
up vote 5 down vote

The orthodox syntax is d[key] = value, but if your keyboard is missing the square bracket keys you could do:

d.__setitem__(key, value)

In fact, defining __getitem__ and __setitem__ methods is how you can make your own class support the dictionary syntax. See http://www.diveintopython.net/object_oriented_framework/special_class_methods.html

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up vote 1 down vote

you can create one

class myDict(dict):

    def __init__(self):
        self = dict()

    def add(self, key, value):
        self[key] = value

## example

myd = myDict()
myd.add('apples',6)
myd.add('bananas',3)
print(myd)

gives

>>> 
{'apples': 6, 'bananas': 3}
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